Calculating Water Footprint of Food
Whilst it is important to consider the total volume of water that is used to produce your food, to look solely the footprint in terms of litres alone generates a somewhat simplified version of the story.
The impact that any food has on the world’s freshwater resources depends not only on the total volume consumed, but also on the form of that water usage, which falls into one of 3 categories:
- Green Water Footprint. Any rainwater utilised – either directly, or from that stored in the soil. Although still calculated into the overall footprint of that food, it is significantly less detrimental to the world’s water resources than the following two categories.
- Blue Water Footprint. Any further water consumption, derived either from surface or groundwater resources.
- Grey Water Footprint. Water which is polluted at any step of the production chain. Rather than referring to water physically consumed in the various production process, the grey water footprint is slightly different to the previous categories. In terms of food, a large proportion of this typically comes from field and farm-run off polluting nearby fresh waterways, as well as in any water used to dilute pollutants produced or used in manufacturing.1,2
Differences Between Water Footprints: Green, Blue, & Grey
Whilst, even if two foods have a seemingly identical water footprint in terms of volume, their impact on water availability and the environment might differ drastically, depending on how that water usage is spread across these three categories.
Blue vs Green Water Footprint: Environmental Damage
Blue and green water consumption both reduce freshwater availability, yet the latter increases the chance of problems such as waterlogging, salinization, soil degradation and water depletion, the repercussions of which can be much wider reaching and longer lasting than reduced water availability alone.3,4 Thus, any food with a predominantly green as opposed to blue water footprint, is likely a more sustainable choice.
Grey Water Footprint: Not Consumed, but No Longer Useable
In contrast to the former two categories, a ‘grey’ water footprint does not have any direct impact on water levels, but rather reduces the quality of that water. This not only has the potential to leave the water unsafe to drink (thus having a comparable impact on water availability), but can also cause widespread damage to the environment with which it comes into contact. The extent of this further damage will depend on a number of factors including what specific contaminants are at play, the concentration of those pollutants, what other uses that water might have had, and where the contaminated water ultimately ends up.5,6
So, which is the “best” water footprint?
Whilst it is pretty clear that, in any instance ‘green’ water usage is preferable over and above blue or grey, determining which of the latter two categories is a more sustainable or ethical choice varies on a case by case basis. A range of variables such as geographical location, season, and the context of the ever-changing climate, all have significant impacts on the water availability in a given location at any given time.
It is for this reason that we often see trade-offs taking place, depending on which type of water-use is considered to be more detrimental in that exact location at that specific time. For example, in order to maximise irrigated crop yields, nitrogen is commonly administered in the form of a fertilizer. Nitrogen is essential for the production of chlorophyll in plants, and greatly enhances photosynthetic capacity to help boost crop yields. Whilst this 'intensification’ increases land productivity and efficiency of water use – thus lowering the blue water footprint - it comes at a cost of drastically increasing the grey water footprint.7,8
Beyond the Numbers
It is for this exact reason why we must look beyond just the numbers. By reducing the blue water component, the food’s overall footprint in terms of litres consumed will drop. That is not to say, however, that in every situation, a lower blue but higher grey water footprint would be the most sustainable, and least damaging choice.
Considering the wider impact of your food choices is, however, easier said than done. Whilst theoretically, we can calculate the impact that any action could, or did have on our environment, when standing in the supermarket aisle faced with hundreds of choices of what to buy for dinner, we cannot conceivably know how one piece of food fares against the next.
There are, however, a few generalised rules which can help you eat better for the environment.